Neuronal Migration Generates New Populations of Neurons That Develop Unique Connections, Physiological Properties and Pathologies

Central nervous system neurons become postmitotic when radial glia cells divide to form neuroblasts. Neuroblastsmay migrate away from the ventricle radially along glia fibers, in various directions or even across the midline. We present four cases of unusual migration that are variably connected to either pathology or formation of new populations of neurons with new connectivities. One of the best-known cases of radial migration involves granule cellsthat migrate from the external granule celllayer along radial Bergman glia fibers to become mature internal granule cells. In various medulloblastomacases this migration does not occur and transforms the external granule celllayer into a rapidly growing tumor. Among the ocular motor neuronsis one unique population that undergoes a contralateral migration and uniquely innervates the superior rectus and levator palpebrae muscles. In humans, a mutation of a single gene ubiquitously expressed in all cells, induces innervation defects only in this unique motor neuronpopulation, leading to inability to elevate eyes or upper eyelids. One of the best-known cases for longitudinal migration is the facial branchial motor (FBM) neurons and the overlapping inner ear efferentpopulation. We describe here molecular cues that are needed for the caudal migration of FBM to segregate these motor neuronsfrom the differently migrating inner ear efferentpopulation. Finally, we describe unusual migration of inner ear spiral ganglionneurons that result in aberrant connections with disruption of frequency presentation. Combined, these data identify unique migratory properties of various neuronal populations that allow them to adopt new connections but also sets them up for unique pathologies.